Known waterborne anti-chipping coatings used for exterior sheet metal parts of vehicles such as automobiles include those comprising a water-based resin dispersion, such as a rubber latex or an acrylic copolymer emulsion, as a vehicle and an inorganic filler, such as calcium carbonate or talc. These coatings are often thickened appropriately and applied to a metal substrate with an airless spray, etc.
A waterborne coating of that type is required to have the following properties. (1) To have an adjustable viscosity for ease of application. (2) To provide a relatively thick coating film in a single pass which dries without developing blisters or cracks. (3) To provide a coating film with excellent adhesion to a sheet metal workpiece (metal substrate). (4) To provide a coating film which has high rubber elasticity thereby exhibiting excellent anti-chipping performance and, if scratched, minimizes corrosion of the underlying metal substrate. (5) To provide a coating film which maintains sufficient impact resistance even under a low temperature condition as in cold climates. (6) To have reusability, which is particularly required in the light of the recent concerns for environmental conservation and the recent severe economic situation. That is, the coating which has adhered to the wall, etc. of a coating booth after coating in a considerable amount should be collected and made reusable by addition of a fresh one. The reusability improves usability of the coating, reduces the amount and/or the concentration of wastewater resulting from coating booth cleaning, and saves the cost for wastewater disposal.
Of the above-described waterborne coatings those based on aqueous acrylic copolymer resin dispersions are disclosed, e.g., in JP-A-53-64287, JP-A-59-75954, JP-A-62-230868, JP-A-63-10678, and JP-A-63-172777.
These waterborne acrylic coatings generally exhibit relatively satisfactory viscosity characteristics and are excellent in airless sprayability, capable of forming a relatively thick coating film in a single pass, and reusable to some extent. Nevertheless many of them cannot be seen as sufficient in adhesion to sheet metal workpieces. In addition, a Pb-Sn alloy-plated steel sheet (called a terne sheet), which has been used as a sheet metal worked substrate in the manufacture of automobiles, etc., has now been regarded problematic in terms of toxicity of lead and being replaced with aluminum-plated steel sheets, tin-zinc-plated steel plates, and the like. Compared with conventional terne sheets, these metal substrates are apt to have insufficient adhesion to coatings. The conventional waterborne coatings based on an acrylic copolymer resin dispersion often show extremely poor adhesion to such metal substrates.
On the other hand, anti-chipping waterborne coatings comprising a rubber latex as a vehicle are disclosed, e.g., in JP-A-57-180617, JP-A-59-75954, and JP-A-59-129213.
The latex-based anti-chipping waterborne coatings described in the publications have difficulty in forming a thick film in a single pass because of their viscosity. Thickening in an attempt to solve the problem results in unfavorable viscosity behavior, which causes difficulty in providing a uniform coating film by spray coating, etc. In particular, the coating film just formed is liable to slip and run on a vertical substrate. Moreover, because the latex-based anti-chipping waterborne coatings often have insufficient mechanical stability, little reusability is expected.
Anti-chipping waterborne coatings using a combination of a rubber latex and other copolymers as a vehicle are also known. For example, JP-A-54-52139 proposes using a vehicle comprising a styrene-butadiene copolymer resin and a specified amount of a vinyl chloride-vinyl acetate-unsaturated dibasic acid copolymer resin and a specified amount of a low-molecular liquid polymer. A working example of the publication uses the vinyl chloride-vinyl acetate-unsaturated dibasic acid copolymer resin in powder form. It has turned out, however, that the copolymer resin powder is not easy to disperse and that the resulting coat has insufficient adhesion to a sheet metal workpiece and insufficient anti-chipping properties.
JP-A-2-28269 proposes using, as a base resin (vehicle component) of an anti-chipping waterborne coating, a copolymer prepared by emulsion polymerization of styrene, butadiene, and an acrylic monomer which has a glass transition temperature of 0° C. or lower and a butadiene content of 5 to 50 parts by weight per 100 parts by weight of the copolymer. According to a working example of the publication, the base resin is prepared by emulsion polymerizing an acrylic monomer in the presence of a styrene-butadiene rubber latex. The inventors of the present invention attempted to prepare the copolymer in accordance with the working example. What they obtained as a result was a rigid and brittle copolymer with no such rubbery elasticity as expected. This is because the emulsion polymerization of an acrylic monomer in the presence of a styrene-butadiene rubber latex is accompanied by graft polymerization of the acrylic monomer to the residual double bonds originated in the butadiene units of the rubber latex.
Application of an anti-chipping coating to automotive parts other than the body, e.g., chassiz, gas tanks, and suspensions, is usually carried out on a separate line from a body coating line. To reduce automobile manufacturing costs, attempts have recently been made to bake the coating film on that line at relatively low temperatures, e.g., 100° C. or lower. The attempts have turned out to be advantageous for practically averting blister development but to give rise another problem that the adhesion of the coating to these parts is reduced further.